This invention relates to a metal film resistor, and more particularly to a fusible metal film resistor.
In prior art, a metal film resistor such as nickel-chromium film resistor and tin oxide film resistor is used widely in various electronic devices as a resistor having high stability and reliability. While such a metal film resistor has a high thermal resistance, it has a deficiency in that when it becomes excessively heated due to overloading it is not easily disconnected. Therefore, such excessively heated resistor may cause burning of ambient combustible materials or destroy an electronic circuit by the overcurrent flowing therethrough.
Recently, a large demand has arisen for safe electronic equipment, and so a resistor which has high performance and reliability at normal load and which fuses easily and instantly at abnormal overload is much desired for preventing fire and destruction of an electronic circuit.
In the prior art, there has been proposed a fusible resistor such as a film resistor on the surface of which a layer of a material having a low melting point is formed. In this case, this layer is fused at overcurrent due to self-heating of the resistor and the resistor is disconnected thereby. However, such a conventional fusible resistor has some problems such as the comparatively long time necessary for fusion and unstable fusing characteristics so that sometimes the resistor film is not completely disconnected. Further, there is another problem in that in manufacturing, the step of forming layer of the material of a low melting point is required which is troublesome and results in a high cost.